I have just discovered that Calibre won't do a restart. When installing a plugin requiring a restart Calibre terminates but doesn't restart. Same behavior if I click on the restart button. I still am very pleased with Calibre. I don't know how any ebook reader could get along without it! I am using Windows 10 and Calibre 3.0 [64 bit]. Thanks again for all the hard work Kovid.
For some reason, unknown to me, the restart is now working. Must be gremlins!